In this video, Prof. Lewin shows an infinitely long wire and a rectangular loop at a distance $r_1$ from each other, each carrying a current. He then calculates the magnetic field resulting from the current in the infinite wire, but ignores the magnetic field generated by the current in the loop. Why can he do this?
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$\begingroup$ Please include the relevant contents of the link in the post itself. $\endgroup$– ACuriousMind ♦Commented Oct 29, 2014 at 18:10
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He "ignores" the magnetic field due to $I_2$ because he is interested only in the effect of the magnetic field caused by $I_1$ on the loop. So while there are indeed internal forces on the loop from the magnetic field that it creates itself, these are by his own admission not the forces he is interested in.
